DEPARTMENT  Ol  TERIOK 

UNITED  STATES  GEOLOGICAL  SURVEY 

• 


PROFESSIONAL  PAPER  90— G 


THE  MONTANA  GROUP  OF  NORTHWESTERN  MONTANA 


BY 


EUGENE   STEBINGER 


'».•(, ,|.,T-    I  J     I!)M 


SHORTER  CONTRIBUTIONS  TO  GENERAL  GEOLOGY^  1914     G 


WASHINGTON 

GOVERNMENT     PRINTING     OFFICE 

1914 


WILLIAM   DILLER   MATTHEW 


GIFT  OF 

WILLIAM  DILLER  MATTHEW 


EARTH 

SCIENCES 
LIBRARY 


DEPARTMENT  OF  THE  INTERIOR 
UNITED  STATES  GEOLOGICAL  SURVEY 

GEORGE  OTIS  SMITH,  DlBECTOK 


PROFESSIONAL  PAPER  90 — G 


THE  MONTANA  GROUP  OF  NORTHWESTERN  MONTANA 


BY 


EUGENE  STEBINGER 


Published  October  14,  1914 


SHORTER  CONTRIBUTIONS  TO  GENERAL  GEOLOGY,  1914— G 


WASHINGTON 

GOVERNMENT    PRINTING    OFFICE 

1914 


MATTHEW 
UBRARY 


CONTENTS. 


Page. 

Introduction 61 

Formations  constituting  th3  Montana  group 61 

Section  on  Two  Medicine  River 61 

Virgelle  sandstone  and  Horsethief  sandstone 62 

Two  Medicine  formation 63 

Bearpaw  shale 64 

Relations  of  the  Montana  group  to  the  Belly  River  and  Judith  River  formations 64 

Relations  of  the  continental  to  the  marine  sediments  in  the  Montana  group 66 


ILLUSTRATION. 


FIGURE  9.  Diagram  showing  probable  relations  of  the  marine  and  continental  sediments  of  the  Montana  group 

in  the  northern  Great  Plains  region,  the  top  of  the  Colorado  group  being  taken  as  a  datum  plane. .         67 
n 


THE  MONTANA  GROUP  OF  NORTHWESTERN  MONTANA. 


By  EUGENE  STEBINGEE. 


INTRODUCTION. 

Recent  field  work  has  shown  that  the  formations  of  the  Montana  group  in  and  near  the 
Blackfeet  Indian  Reservation  of  northwestern  Montana  are  very  different  from  those  of  the 
type  area  of  the  Judith  River  and  associated  formations  on  Missouri  and  Musselshell  rivers, 
in  the  central  part  of  the  State,  but  similar  to  the  formations  in  southern  Alberta  as  described 
by  Dawson.  They  seem  to  deserve  special  description  not  only  because  they  present  new 
evidence  regarding  the  relations  of  the  Belly  River  formation  of  Canada  to  the  Judith  River 
formation,  but  also  because  they  indicate  the  varying  geographic  conditions  that  prevailed 
during  Cretaceous  time  in  this  region.  Furthermore,  the  stratigraphy  as  interpreted  affords 
an  excellent  example  of  variation  in  sedimentation  from  the  seaward  to  the  landward  side  of  a 
zone  of  deposition. 

The  Montana  group,  as  here  described,  was  first  studied  by  the  writer  in  the  summer  of 
1911,  along  Cut  Bank  Creek  and  Two  Medicine  River  on  the  Blackfeet  Indian  Reservation, 
in  company  with  Mr.  T.  W.  Stanton,  who  has  already  briefly  described  some  of  the  features 
of  this  region  *  and  to  whom  the  writer  is  much  indebted  for  paleontologic  determinations 
which,  especially  those  for  the  marine  formations  encountered,  were  of  great  assistance  in 
interpreting  the  stratigraphy.  During  the  next  two  seasons  these  formations  were  traced 
northward  into  the  type  area  of  the  Belly  River  beds,  described  by  G.  M.  Dawson  2  in  1885, 
and  eastward  into  an  area  of  the  Judith  River  formation,  as  definitely  determined  by  Stanton 
and  Hatcher3  in  1903.  The  interpretation  of  the  field  relations  presented  is  therefore  based 
primarily  on  the  area!  tracing  of  the  formations,  although  every  conclusion  was  substantiated 
by  paleontologic  evidence. 

FORMATIONS  CONSTITUTING  THE  MONTANA  GROUP. 
SECTION    ON    TWO    MEDICINE    RIVER. 

A  section  showing  the  complete  sequence  of  the  formations  of  the  Montana  group  in  north- 
western Montana  is  exposed  along  the  valley  of  Two  Medicine  River  in  the  southern  part  of  the 
Blackfeet  Indian  Reservation,  and  because  of  its  completeness  and  of  the  excellence  of  the 
exposures  is  offered  as  a  standard  for  the  region.  The  section  extends  westward  upstream 
from  the  mouth  of  the  river,  hi  T.  31  N.,  R.  5  W.,  to  a  point  about  3  miles  above  the  Holy 
Family  Mission  (Family  post  office),  in  T.  31  N.,  R.  9  W.  Throughout  this  distance,  about  3") 
miles,  the  formations  either  lie  nearly  flat  or  dip  slightly  westward  at  an  angle  rarely  exceeding 
3°.  As  the  river  runs  nearly  at  a  right  angle  to  the  strike,  it  crosses  the  formations  in  succession 
from  younger  to  older.  On  the  whole  the  locality  is  almost  ideal  for  a  study  of  the  formations, 
the  structure  being  simple  and  easily  determined  and  the  rocks  perfectly  exposed  throughout 
most  of  the  distance  in  the  minutely  carved  badlands  along  the  sides  of  the  valley. 

1  Stanton,  T.  W.,  Some  variations  in  Upper  Cretaceous  stratigraphy;  presidential  address  before  the  Geological  Society  of  Washington,  1912: 
Washington  A  cad.  Sci.  Jour.,  vol.  3,  pp.  66-69,  1913. 

2  Ueport  on  the  region  in  the  vicinity  of  the  Bow  and  Belly  rivers,  Northwest  Territory:  Canada  Geol.  Survey  Kept.  Progress  for  1882-1884, 
pp.  1-169C. 

3  Stanton,  T.  \V.,  and  Hatcher,  J.  B.,  Geology  and  paleontology  of  the  Judith  Kiverl-eds,  with  a  chapter  on  the  fossil  plants  by  F.  II.  .:  ,wlton: 
U.  S.  Geol.  Survey  Bull.  257,  pp.  51-53,  1905. 

54476°— 14  61 

767418 


62 


SHORTER   CONTRIBUTIONS   TO   GENERAL   GEOLOGY,   1914. 


From  the  top  of  the  Colorado  shale,  which  is  readily  recognized  by  its  characteristic  marine 
fauna  and  general  appearance,  the  rocks  naturally  group  themselves  in  ascending  order  into 
four  lithologic  units — (1)  a  massive  gray  sandstone,  chiefly  marine;  (2)  a  rudely  bedded  mass 
of  clay  and  sandstone,  chiefly  a  fresh-water  deposit;  (3)  a  dark  marine  clay  shale;  and  (4)  a 
massive  gray  sandstone,  brackish  and  marine.  Nos.  1  and  3  correspond  to  the  Virgelle  sand- 
stone, as  the  lower  sandstone  member  of  the  Eagle  sandstone  has  recently  been  designated,1 
arid  to  the  Bearpaw  shale  of  the  Missouri  and  Musselshell  river  section  in  central  Montana, 
•whereas  Nos.  2  and  4,  formations  hitherto  not  recognized,  whose  stratigraphic  equivalents 
elsewhere  are  represented  by  formations  having  different  limits  and  lithology,  are  here  named 
the  Two  Medicine  formation  and  Horsethief  sandstone.  The  important  details  of  the  section 
are  shown  below. 

Formations  of  the  Montana  group  of  northwest  Montana  exposed  on  Two  Medicine  River  between  its  mouth  and  Family 

post  office. 


Formation. 

Lithology. 

Fossils. 

Topographic  features. 

Thick- 
ness in 

feet. 

Horsethief  sandstone. 

Gray  to  buff  coarse-grained,  much 
cross-bedded,  massive  sandstone 
in  upper  half.  In  lower  half 
slabby  gray  sandstone,  becom- 
ing shaly  toward  the  base.  In 
places  beds  of  heavy  magnetite- 
bearing  sandstone. 

Mainly  a  brackish-water  fauna 
containing  Ostrea;  Corbicula, 
Corbula,and  Anomia.  In  places 
a  marine  littoral  fauna  yielding 
Tancredia  americana,  Cardium 
speciosum,  and  Mactra. 

Where  the  formation  lies  nearly 
horizontal  it  may  form  a  bold 
escarpment;  where  it  is  steeply 
tilted  it  forms  prominent  hog- 
back ridges. 

360 

Bearpaw  shale. 

Dark-gray  clay  shale  with  a  few 
limestone  concretions. 

An  abundant  marine  fauna 
(Pierre)  containing  Baculites, 
Placenticeras,Inoceramus,  Avic- 
ula,  and  Scaphites. 

Subdued  and  rounded  topog- 
raphy, gumbo  soil,  and  few 
exposures. 

-190 

Two  Medicine  formation. 

Gray  to  greenish-gray  clay  and 
soft  irregular  sandstone  which  is 
most  abundant  in  the  lower  250 
feet.  In  places  thin  beds  of  red 
clay  and  nodular  limestone. 

An  abundant  reptilian  fauna  of 
Judith  River  types.  At  many 
horizons  a  fresh-water  fauna 
containing  Unio,  Viviparus.  and 
Campeloma;  at  others  brackish- 
water  forms,  Ostrea,  Corbula, 
etc.  One  marine  horizon  yield- 
ing Tancredia  americana,  Car- 
dium speciosum,  Mactra,  land 
plants,  and  much  fossil  wood. 

Eroded  into  extensive  badlands 
along  principal  streams;  else- 
\\  here  a  smooth  rounded  topog- 
raphy. 

1,050 

Virgelle  sandstone. 

Gray  to  buff  coarse-grained,  much 
cross-bedded,  massive  sandstone 
with  many  ferruginous  concre- 
tions in  upper  half.  In  lower 
half  slabby  gray  sandstone,  be- 
coming shaly  toward  the  base. 

Contains  a  poorly  developed  lit- 
toral marine  fauna. 

Where  the  formation  lies  nearly 
horizontal  it  forms  a  bold  es- 
carpment in  many  places; 
where  it  is  steeply  tilted  it 
forms  prominent  hogback 
ridges. 

220 

VIRGELLE    SANDSTONE    AND    HORSETHIEF    SANDSTONE.  •« 

Both  these  formations  are  composed  of  massive  gray  to  buff  sandstone,  and  although  they 
are  2,500  feet  apart  stratigraphically  they  are  so  nearly  identical  that  they  can  best  be  described 
together.  Each  seems  to  be  a  sandy  beach  or  near-shore  deposit  laid  down  in  a  retreating  sea, 
the  Virgelle  representing  a  great  recession  of  the  Colorado  sea  at  the  close  of  Colorado  time 
and  the  Horsethief  representing  an  even  greater  recession  at  the  close  of  Pierre  time.  The  two 
are  therefore  genetically  identical,  each  being  a  sandstone  laid  down  on  a  marine  shale  in  a 
retreating  sea.  Similar  rocks  are  associated  with  both  of  these  formations,  each  being  overlain 

1  Unpublished  report  by  C.  F.  Bowen  entitled  "The  stratigraphy  of  the  Montana  group"  (U.  S.  Geol.  Survey  Prof.  Paper  90-1),  from  which 
the  following  definition  is  quoted:  "The  name  Eagle  sandstone  was  given  by  Weed  (U.  S.  Geol.  Survey  Geol.  Atlas,  Fort  Benton  folio,  No.  55) 
to  the  formation  overlying  the  Colorado  shale  in  north-central  Montana  and  typically  exposed  on  Missouri  River  at  the  mouth  of  Eagle  Creek,  40 
miles  below  Fort  Benton.  In  the  type  locality  the  formation  as  de'ined  by  Weed  consists  of  three  more  or  less  distinct  units,  comprising  an 
upper  member  of  thin-bedded  sandstone,  a  middle  member  of  shale,  and  a  lower  member  of  massive  ledge-making  sandstone.  This  lower  mem- 
ber is  so  persistent  and  characteristic  over  a  large  area  in  north-sentral  Montana,  even  whe-e  the  other  divisions  of  the  formation  are  not  r3:og- 
niza!  le,  that  it  seems  desiralle  for  purposes  of  description  and  correlation  to  give  it  a  name.  It  is  well  exposed  along  Missouri  P.iverfrcm 
the  town  of  Virgelle,  a  fe'.v  miles  below  Fort  Benton,  eastward,  and  the  name  Virgelle  sandstone  member  of  the  Eagle  sandstone  has  thererore 
been  adopted  by  the  United  States  Geological  Survey  for  this  division  of  the  formation.  The  Virgelle  sandstone  member  is  the  lower  missive 
ledge-making  sandstone  of  the  Eagle  sandstone  as  defined  by  Weed."  In  northwestern  Montana  the  Virgelle  sandstone  is  recognized  as  a  dHinct 
formation,  and  the  overlying  Two  Medicine  formation  includes  the  equivalent  of  the  remainder  of  the  Eagle  sandstone. 


THE   MONTANA   GROUP   OF    NORTHWESTERN    MONTANA.  63 

by  a  thick  mass  of  continental  deposits,  apparently  conformable  and  very  similar  in  compositio 
and  appearance. 

In  the  section  along  Two  Medicine  River  the  Virgelle  sandstone  rises  in  bold  cliffs  above 
the  few  feet  of  sandy  shale  that  form  the  uppermost  beds  of  the  Colorado  group  exposed 
at  the  mouth  of  the  river.  From  this  point  the  sandstone,  dipping  very  slightly  westward,  is 
continuously  exposed  upstream  for  a  distance  of  8  miles,  forming  a  narrow,  steep-walled  canyon 
several  hundred  feet  deep,  which  is  a  little-known  but  picturesque  feature  of  this  part  of  Mon- 
tana. Similarly  the  Horsethief  sandstone,  where  it  is  cut  through  by  Two  Medicine  River  just 
above  Family  post  office,  also  stands  out  in  bold  cliffs  over  300  feet  high. 

The  principal  fossils  found  in  these  sandstones  are  invertebrates,  although  in  manv  places 
both  the  formations,  especially  the  Virgelle,  seem  barren.  The  Horsethief  sandstone  (named 
for  Horsethief  Ridge,  in  the  Blackfoot  quadrangle)  has  yielded  both  marine  and  brackish  water 
forms.  The  brackish  water  forms  are  mainly  oysters,  which  are  locally  very  abundant;  the 
marine  forms  belong  to  the  Fox  Hills  near-shore  fauna,  the  dominant  species  being  Tancredia 
americana  and  Cardium  speciosum.  On  Two  Medicine  River  the  Virgelle  sandstone  is  appar- 
ently unfossiliferous,  but  elsewhere  it  is  known  to  contain  a  marine  near-shore  fauna. 

TWO    MEDICINE    FORMATION. 

The  Two  Medicine  formation,  which  lies  immediately  above  the  Virgelle  sandstone,  is  a 
great  mass  of  light-colored  rocks  about  2,000  feet  thick,  consisting  mainly  of  typical  conti- 
nental deposits.  From  the  upper  end  of  the  canyon  in  the  Virgelle  sandstone,  in  T.  31  N., 
R.  5  W.,  nearly  all  the  formation  is  excellently  exposed  in  badlands  that  extend  for  20  miles 
upstream  to  the  mouth  of  Badger  Creek,  in  T.  31  N.,  R.  8  W.  It  is  composed  principally  of 
light-gray  to  greenish-gray  clay  and  clay  shale,  so  rudely  bedded  that  it  is  impossible  to  follow 
a  -riven  stratum  for  any  great  distance.  In  places  the  beds  of  clay  are  variegated,  red  and 
yellow  strata  appearing.  Thin  nodular  and  nonpersistent  limestone,  apparently  of  fresh- 
water origin,  also  occurs  at  irregular  intervals.  Probably  20  per  cent  of  the  total  mass  of  the 
formation  is  made  up  of  soft  coarse-grained  sandstone  in  lenticular  beds  which,  even  where  20 
to  30  feet  thick,  can  not  be  traced  more  than  1  or  2  miles.  At  many  localities  these  sandstones 
show  the  very  irregular  cross-bedding  that  is  characteristic  of  eolian  deposits.  The  lower  200 
feet  of  the  formation  is  more  sandy  than  the  remainder,  probably  half  of  this  part  consisting  of 
massive  sandstone  in  irregular  beds,  the  thickest  measuring  50  feet. 

A  well-developed  vertebrate  fauna,  chiefly  dinosaurs  of  Judith  River  types,  occurs  in  this 
formation.  The  abundance  of  fossil  bones,  especially  in  the  upper  200  feet,  is  unusual.  C.  W. 
Gilmore,1  of  the  United  States  National  Museum,  who  spent  the  season  of  1913  in  collecting 
from  these  beds,  noted  parts  of  13  different  individuals  of  dinosaurs  in.  the  course  of  a  single 
cluv's  field  work.  Much  of  the  material  collected  by  him  is  new  and  will  add  greatly  to  our 
knowledge  of  the  Cretaceous  reptiles.  Fragments  of  fossil  wood  and  many  complete  sections  of 
tree  trunks  are  also  very  common  throughout  the  formation.  All  the  plant  remains  collected 
were  identified  by  F.  II.  Knowlton  as  "  Belly  River  or  Judith  River." 

The  fossil  shells  found  in  the  Two  Medicine  formation  also  indicate  that  it  is  essentially  of 
continental  origin.  Unio,  Viviparus,  and  other  fresh-water  genera  being  found  at  many  horizons. 
In  the  lower,  sandy  part  of  the  formation,  as  well  as  at  the  top,  there  are  local  thin  brackish- 
water  beds  with  Ostrea,  Corbula,  and  Corbicula.  A  fauna  indicating  a  temporary  incursion  of 
purely  marine  waters  while  the  formation  was  being  deposited  occurs  in  a  sandstone  about  200 
feet  above  its  base  and  has  already  been  characterized  by  Mr.  Stanton2  as  a  Claggett-Fox  Hills 
fauna.  It  is  almost  identical  with  the  fauna  found  at  the  top  of  the  Horsethief  sandstone,  a,  fact 
that  shows  that  it  existed  in  the  shallow-water  portions  of  the  Pierre  sea  throughout  neasly 
all  Montana  time. 


1  Personal  communication. 

2  Stanton,  T.  W.,  Some  variations  in  Upper  Cretaceous  stratigraphy:  Washington  Ai-ad.  Sci.  Jour.,  vol.  3,  p.  (>6,  1913. 


64  SHORTER  CONTRIBUTIONS   TO   GENERAL   GEOLOGY,  1S14. 

BEARPAW    SHALE. 

The  Bearpaw  shale  is  a  typical  dark  marine  clay  shale  averaging  500  feet  in  thickness  in 
this  part  of  the  State.  The  change  in  the  character  of  the  rocks  from  the  Two  Medicine  forma- 
tion to  this  one  is  very  abrupt,  there  being  practically  no  transition  beds  near  the  contact,  and 
although  shallow  water  must  have  existed  for  some  time  at  this  horizon  while  the  Bearpaw  sea 
was  advancing,  no  trace  of  the  marine  fauna  of  Fox  Hills  facies,  noted  in  the  Two  Medicine 
formation  and  above  in  the  Horsethief  saridstone,  has  yet  been  found  here.  Invertebrate 
fossils  are  abundant  in  concretions  throughout  the  shale  and  are  of  characteristic  Pierre  types, 
allowing  fairly  close  and  certain  correlation  with  the  Bearpaw  shale  of  central  Montana. 

RELATIONS   OF  THE    MONTANA  GROUP    TO    THE  BELLY  RIVER   AND  JUDITH 

RIVER  FORMATIONS. 

The  age  of  the  Judith  River  formation  and  its  correlation  with  the  Belly  River  formation 
of  Canada  have  been  under  discussion,  mainly  by  paleontologists,  ever  since  the  Judith  River 
beds  were  first  studied.  Nearly  every  worker  who  during  the  last  40  years  has  been  interested 
in  the  Cretaceous  formations  of  the  Rocky  Mountain  and  Great  Plains  regions  has  taken  part 
in  this  discussion,  the  extensive  literature  of  which  has  been  thoroughly  reviewed  by  Stanton 
r.:id  Hatcher.1  In  1902,  on  the  publication  of  Lambe  and  Osborn's  report 2  on  the  vertebrate 
fauna  of  the  Belly  River  formation,  the  controversy  3  was  revived  with  renewed  vigor.  So 
great  a  difference  of  opinion  was  developed  during  the  following  winter  that  Stanton  and 
Hatcher  decided  on  a  joint  field  investigation,  which  they  made  in  the  summer  of  1903.  The 
results  of  their  work  were  published  first  in  a  preliminary  note  4  and  later  in  a  detail  report. 
They  determined  the  succession  of  formations  in  the  Montana  group  for  the  central  Montana 
region,  which  has  since  been  found  to  extend  over  a  large  area,  and  proved  the  Cretaceous 
age  of  the  Judith  River  formation.  Then,  by  means  of  practically  continuous  exposures,  they 
traced  the  Judith  River  formation  into  the  extreme  southeastern  part  of  the  large  area  of 
Belly  River  beds,  as  originally  mapped  by  G.  M.  Dawson.5  This  led  to  their  second  impor- 
tant conclusion,  namely,  that  the  Belly  River  formation  and  the  Judith  River  formation  are 
of  identical  age.  Their  conclusions  were  generally  accepted  as  having  satisfactorily  answered 
this  much-discussed  question  in  Cretaceous  stratigraphy.  There  was  no  reason  to  suspect  that 
the  type  section  of  the  Belly  River  beds  as  described  by  Dawson,  occurring  on  Belly  River, 
was  different  from  that  in  the  locality  into  which  they  had  traced  the  Judith  River  forma- 
tion. That  these  sections  are  different  appeared  only  when  the  section  here  described  was 
examined  and  compared  both  with  Dawson's  section  and  with  the  section  of  Stanton  and 
Hatcher  on  Missouri  River. 

In  a  recent  paper  6  A.  C.  Peale  again  raises  the  question  of  the  age  of  the  Juditft  River 
formation.  He  does  not  deny  the  existence  of  the  formations  of  the  Montana  group  in  central 
Montana  in  the  succession  as  originally  described  by  Stanton*  and  Hatcher,  nor  that  they 
extend  over  a  large  area  Li  central  Montana,  as  proved  by  C.  F.  Bowen,  R.  W.  Stone,  and 
other  workers,  but  contends  that  beds  originally  called  Judith-  River  in  the  type  area  on  the 
Missouri  near  the  mouth  of  Judith  River  are  of  Lance  age  and  are  not  equivalent  to  the  forma- 
tions mapped  as  Judith  River  over  a  large  part  of  Montana.  Peale's  contention  thus  raises  the 
question  of  the  exact  interpretation  of  the  structure  and  stratigraphy  in  the  small  disturbed 
type  area  at  the  mouth  of  Judith  River,  and  if  his  view  is  substantiated  the  name  Judith  River, 
as  applied  to  the  formation  which  has  been  so  extensively  mapped  under  that  name,  will  be 

i  Stanton,  T.  W.,  and  Hatcher,  J.  B.,  Geology  and  paleontology  of  the  Judith  River  beds:    U.  S.  Geol.  Survey  Bull.  257,  pp.  14-31,  1905. 

*  On  vertebrate  of  the  mid-Cretaceous  of  the  Northwest  Territory:  Lambe,  L.  M.,  New  genera  and  species  from  the  Belly  River  series  (mid- 
Cretaceous);  Osbom,  II.  F.,  Distinctive  characters  of  the  mid-Cretaceous  fauna:  Canada  Geol.  Survey.  Contr.  Canadian  Paleontology,  vol.  3,  pi.  2, 
1902. 

3  Science,  new  r  r.,  vols.  16,  17,  18. 

<  Hatcher,  J.  B.,  and  Stanton,  T.  W.,  The  stratigraphic  position  of  the  Judith  River  beds  and  their  correlation  with  the  Belly  River  beds: 
Science,  near  sen,  vol.  18,  pp.  211-212, 1903. 

5  Canada  Geol.  Survey  Rept.  Progress  for  1882-18&4,  pp.  1-169C. 

«  Jour.  Geology,  vol.  20,  pp.  530-549. 


THE   MONTANA   GEOUP   OF   NORTHWESTERN   MONTANA. 


65 


invalidated.  Therefore,  without  entering  into  the  question  raised  by  Peale,  it  is  important  to 
insist  that  the  correlations  made  later  in  this  paper  refer  to  the  formation  mapped  as  Judith 
River  oven-  nearly  alt  of  central  Montana. 

In  the  course  of  their  stratigraphic  study  in  central  Montana,  Stan  ton  and  Hatcher  estab- 
lished the  succession  shown  in  the  following  table : 

Section  of  Cretaceous  rocks  in  central  Montana. 


Formal  ion. 

Character  of  rocks  and  fossils. 

Thick- 
ness in 
feet. 

Bearpaw  shale. 

Dark  clay  shale  with  marine  fauna  

750? 

Judith  River  formation  
Claggett  formation 

Variable  shale  and  soft  sandstone  with  dinosaurs  and  other  reptiles  and  fresh  and  brackish  watershells.  . 
Dark  clay  shale  with  several  beds  of  sandstone,  especially  in  the  upper  part;  marine  fossils  throughout 

500 
400± 

Eagle  sandstone  .  . 

those  of  the  sandstone  including  many  Fox  11  ills  species. 
White,  gray  ,  and  yellowish  massive  sandstone  in  lower  and  shale,  coal,  and  thinsandstone  inupper  part. 

200-300 

Colorado  shale 

Contains  marine  shells,  land  plants,  and  dinosaurs. 
Dark  marine  shale 

800+ 

On  comparing  this  section  with  that  on  Two  Medicine  River,  the  Colorado  shale,  which 
can  be  traced  continuously  from  its  outcrops  on  Missouri  River  to  those  at  the  mouth  of  the 
Two  Medicine  and  is  also  easily  recognized  because  of  its  characteristic  marine  fauna  and 
appearance,  affords  a  safe  stratigraphic  datum  plane  on  which  to  base  comparative  sections. 
First  of  all,  the  identification  of  the  sandstone  lying  above  the  Colorado  shale  in  the  Two  Medicine 
section  as  the  lower  or  Virgelle  sandstone  member  of  the  Eagle  is  made  fairly  certain  by  both 
its  position  and  the  fact  that  it  also  can  be  traced  on  the  outcrop  over  the  greater  part  of  the 
distance  from  the  exposures  on  Two  Medicine  River  to  those  on  the  Missouri  at  the  type  locality, 
though  its  limits  may  vary  slightly  from  point  to  point.  The  outcrop  of  this  sandstone  can  be 
traced  without  interruption  in  bold  cliff  exposures  from  the  mouth  of  Two  Medicine  River  to 
the  Sweetgrass  Hills.  From  this  locality  to  the  outcrops  of  the  Eagle,  near  Virgelle,  on  the 
Missouri,  the  bedrock  formations  are  completely  covered  with  drift,  but  the  continuity  of  the 
sandstone  over  all  of  this  distance  is  suggested  by  its  presence  in  a  well  boring  in  T.  34  N.,  R.  8  E. 
The  formation  recognized  as  Bearpaw  on  Two  Medicine  River  can  not  be  traced  to  Missouri 
River  in  the  same  manner  because  it  has  been  eroded  away  between  these  localities,  but  fortu- 
nately a  characteristic  marine  fauna  is  found  in  the  shale,  which,  together  with  the  lithologic 
appearance,  thickness,  and  relative  position  of  the  formation  above  the  Colorado  shale,  makes 
its  correlation  with  the  Bearpaw  shale  of  the  Missouri  River  section  about  as  certain  as  a  corre- 
lation of  this  sort  can  be.  This  leaves  the  Two  Medicine  formation  occupying  the  entire  interval 
between  the  Virgelle  sandstone  and  the  Bearpaw,  and  it  is  therefore  equivalent  to  all  the  strata 
on  Missouri  River  that  make  up  the  Judith  River  formation,  the  Claggett  formation,  and  the 
coal-bearing  shale  and  sandstone  forming  the  upper  member  of  the  Eagle  sandstone. 

In  his  report  on  the  region, in  the  vicinity  of  the  Bow  and  Belly  rivers  Dawson  *  established 
the  following  section  for  the  Cretaceous  rocks  occurring  there : 

Section  of  Cretaceous  rocks  near  Bow  and  Belly  rivers,  Canada. 


Formation. 

Character  of  rocks. 

Thick- 
ness in 
feet. 

Fox  Hills  sandstone. 

In  some  parts  of  the  district  well  defined  as  a  massive  yellowish  sandstone,  tut  inctrstant  and  appar- 
ently often  represented  by  a  series  of  brackish-water  transition  beds  between  the  La.rarr.ie  and  Pierre. 
Neutral  gray  or  brownish  to  nearly  black  shales,  include  a  zone  of  pale  soft  sandsirne  in  the  north- 
eutern  part  of  district  and  frequent  intercalations  of  harder  sandal  ones  near  tbe  mountains.    Marine. 
Composed  of  an  upper  or  "pale"  and  lower  or  "yellowish"  portions,  and  consi:ting  of  alternations  of 
sandstones,  sandy  clays,  shales,  and  clays. 
Gray  to  nearly  black  shales  in  places  including  arenaceous  shales  

so 
750 
910 
800 

Pierre  shales 

Belly  River  series  

This  section  was  compared  with  the  section  on  Two  Medicine  River  and  it  was  fortunately 
found  possible  to  trace  all  the  members  of  the  Two  Medicine  section  due  north  to  the  equivalent 
parts  of  the  section  in  Alberta  without  interruption  in  the  outcrops.  The  Colorado  shale 


i  Canada  Geol.  Survey  Kept.  Progress  for  1882-1884,  p.  112-c. 


66  SHORTER  CONTRIBUTIONS   TO   GENERAL   GEOLOGY,  1914. 

exposed  at  the  mouth  of  Two  Medicine  River  can  be  traced  directly  into  an  area  near  the  inter- 
national boundary  mapped  by  Dawson  as  Lower  dark  shales.  The  Bearpaw  shale  can  be  traced 
with  equal  facility  into  the  shale  mapped  by  Dawson  as  Pierre,  and  the  Horsethief  sandstone 
into  Dawson's  Fox  Hills  sandstone.  The  strata  designated  by  Dawson  the  Belly  River  series 
comprise  all  the  rocks  between  his  Lower  dark  shales  and  Pierre  shales,  so  that  if  his  Lower 
dark  shales  are  Colorado  shale,  and  his  Pierre  shale  is  Bearpaw,  it  becomes  evident  that  his  Belly 
River  series,  lying  between  these  two  marine  shales,  is  equivalent  to  the  Virgelle  sandstone 
and  Two  Medicine  formation  taken  together/ for  they  lie  between  the  same  two  marine  shales 
and  the  formations  are  entirely  conformable;  or,  carrying  the  correlations  to  Stan  ton  and 
Hatcher's  section  on  Missouri  River,  it  is  apparent,  from  the  correlation  between  the  section  on 
Two  Medicine  River  and  that  on  the  Missouri,  already  made,  that  the  Belly  River  formation 
of  Canada  is  equivalent  to  the  .Eagle,  Claggett,  and  Judith  River  formations  combined. 

The  correlations  made  by  Stanton  and  Hatcher  in  1903,  by  tracing  the  Judith  River  forma- 
tion up  Milk  River  into  Canada  to  beds  mapped  as  Belly  River  by  Dawson,  leading  them  to  the 
conclusion  that  the  Belly  River  and  Judith  River  formations  were  identical,  are  correct  as  far  as 
they  had  gone,  as  can  be  understood  by  reference  to  the  diagram  (fig  9).  Starting  from  the  sec- 
tion in  central  Montana,  they  traveled  northwestward,  following  the  wedge  of  the  Judith  River 
formation  to  a  point  on  Milk  River  near  Pakowki  Lake  in  Alberta,  where  the  section  is  as  at 
A  in  the  accompanying  diagram  and  there  found  Claggett  shale  beneath  the  Judith  River 
formation.  But  Dawson  had  already  mapped  the  beds  found  in  this  locality  as  Belly  River, 
so  that  Stanton  and  Hatcher  were  justified  in  assuming  the  identity  of  the  Belly  River  and 
Judith  River  formations.  Dawson  had  also  seen  and  described  the  dark  marine  shale  of  the 
Claggett  near  Pakowki  Lake,  but  had  erroneously  correlated  it  with  his  Lower  dark  shales 
(Colorado  shale)  of  the  area  he  mapped  farther  west,  so  that  the  formation  he  had  mapped  as 
Belly  River  in  the  vicinity  of  Pakowki  Lake  included  only  a  part  of  the  Belly  River  formation 
as  he  had  defined  it  in  his  type  area  along  Belly  River. 

Dawson's  error  in  correlating  beds  in  different  parts  of  the  area  originally  mapped  as  Belly 
River,  although  suspected  by  Stanton  and  Hatcher,1  has  apparently  never  been  recognized  by 
Canadian  geologists,  for  since  the  publication  of  Stanton  and  Hatcher's  report  they  have  assumed 
that  the  section  of  the  Montana  group  occurring  on  the  Missouri  in  central  Montana  was  appli- 
cable to  all  the  area  of  southern  Alberta  up  to  the  base  of  the  Rockies.  This  seems  evident  from 
a  report  by  D.  D.  Cairnes,  of  the  Canadian  Geological  Survey,  on  the  Moose  Mountain  district,2 
a  tract  lying  adjacent  to  the  mountain  front  in  the  "disturbed  belt"  on  the  west  edge  of  the 
large  area  originally  mapped  by  Dawson.  Cairnes  has  described  Claggett  shale  from  this 
district,  but,  reasoning  from  our  present  understanding  of  the  relations  between  the  Judith 
River  and  Claggett  formations  to  the  beds  mapped  by  Dawson  as  Belly  River,  it  seems  highly 
improbable  that  a  lithologic  unit  equivalent  to  the  Claggett  shale  exists  at  any  point "^n  the 
western  part  of  the  area  originally  mapped  by  Dawson.  Cairnes,  because  of  the  intricate  struc- 
ture of  the  foothill  belt  near  the  "mountains,  has  probably  mistaken  areas  of  "Bearpaw  shale  for 
the  Claggett,  to  which  it  is  very  similar  both  lithologically  and  in  fossil  content. 

RELATIONS    OF    THE    CONTINENTAL    TO    THE     MARINE    SEDIMENTS    IN    THE 

MONTANA   GROUP. 

The  formations  at  the  locality  on  Two  Medicine  River  described  in  this  paper  lie  near  the 
western  edge  of  the  Cretaceous  interior  basin.  (See  index  map  on  fig.  9,  showing  the  Cretaceous 
deposits  of  the  interior  basin  of  North  America.)  No  areas  west  of  this  locality  afford  exposures 
of  the  Montana  group  belonging  to  this  basin,  so  that  by  comparing  sections  of  the  Montana 
from  the  Black  Hills  region,  the  central  Montana  region,  and  the  section  on  Two  Medicine  River, 
all  of  which  lie  approximately  on  a  straight  line,  we  are  able  to  present  correlations  of  this  part 
of  the  Cretaceous  from  a  point  near  the  center  of  the  Cretaceous  basin  to  a  point  on  the  western 


1  Stanton,  T.  W.,  and  Hatcher,  J.  B.,  Geology  and  paleontology  of  the  JudUh  River  beds:  U.  S.  Geol.  Survey  Bull.  257,  pp.  24-25,  1905. 

2  Moose  Mountain  district  of  southern  Alberta:  Canada  Geol.  Survey  Bull.  968,  pp.  28-29,  1907. 


THE    MONTANA   GROUP   OF    NORTHWESTERN    MONTANA. 


67 


edge  of  the  strata  as  they  now  re- 
main. In  other  words,  we  can  show 
the  relations,  definitely  in  some  areas 
and  with  probability  in  others,  of 
the  sediments  that  must  have  been 
deposited  near  the  center  of  this 
Montana  sea  to  those  along  its  west- 
ern margin. 

These  relations  are  illustrated 
in  the  diagram  forming  figure  9. 
The  section  at  each  of  the  localities 
given  is  built  up  on  the  top  of  the 
Colorado  group  as  a  datum  plane, 
the  rock  units  being  all  conformable 
and  the  thicknesses  drawn  to  scale, 
so  that  the  relative  thickening  or 
thinning  of  these  units  is  properly 
indicated.  The  sediments  that  are 
mainly  continental  are  indicated  by 
open  lining,  those  that  are  mainly 
marine  are  shown  in  closer  lining, 
and  the  three  important  sandstone 
units  of  the  group  are  shown  in  a  stip- 
pled pattern .  The  general  thickening 
of  the  strata  on  the  landward  side 
of  the  basin,  due  to  the  increase  in 
the  supply  of  material,  is  evident  at 
a  glance.  The  record  from  the  top 
of  the  Colorado  shale,  a  horizon  that 
probably  marks  the  period  of  maxi- 
mum extent  of  the  Upper  Cretaceous 
sea  in  this  region,  shows  a  series  of 
advances  and  retreats  of  the  sea  on 
the  westward  or  landward  side  of 
this  basin,  while  marine  conditions 
to  the  east  or  seaward  side  remained 
undisturbed.  These  were,  as  indi- 
cated by  arrows  on  the  diagram, 
first,  a  marked  recession  of  the  sea 
during  which  the  Virgelle  sandstone 
was  laid  down,  followed  immedi- 
ately by  the  piling  up  of  continen- 
tal sediments  which  in  the  sec- 
tion on  the  Two  Medicine  are  repre- 
sented by  the  strata  at  the  base  of 
the  Two  Medicine  formation;  next 
an  advance  of.  the  sea  which  did 
not  reach  so  far  as  Two  Medicine 
River  except  in  probably  only  one 
brief  epoch,  during  which  the  wedge 
of  the  marine  Claggett  shale  was 
deposited;  next  a  second  recession, 
during  which  again  a  shore  sand — 


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68  SHOBTEE  CONTRIBUTIONS   TO   GENERAL   GEOLOGY,  1914. 

the  upper  part  of  the  Claggett — was  laid  down.  This  recession  was  immediately  followed  by 
a  period  of  continental  deposition,  in  which  was  formed  the  wedge  of  the  Judith  River 
formation;  then  a  second  advance — the  Bearpaw — much  more  extensive  than  the  Claggett, 
which,  like  the  former  advance,  did  not  result  in  the  deposition  of  any  great  amount  of  sand; 
and,  lastly,  a  third  and  very  extensive  recession,  probably  the  final  one  for  the  western  part 
of  the*  region  considered,  during  which  again  a  persistent  and  easily  identifiable  sandstone  was 
laid  down,  this  deposition  being  followed  immediately  by  the  piling  up  of  continental  sediments. 
The  deposition  of  a  sand  upon  each  retreat  of  the  sea  in  contrast  to  the  practical  absence  of  such 
a  sand  on  each  advance  is  a  notable  phenomenon  in  this  sedimentary  record,  and  seems  to  indi- 
cate that  each  retreat  of  the  sea  was  accompanied  by  land  uplift  with  an  increase  in  stream 
gradients  and  consequent  coarsening  of  sediments  and  each  advance  by  the  opposite  conditions. 
The  close  similarity  between  these  three  sandy  formations  is  further  brought  out  by  the  fact 
that  they  contain  littoral  marine  faunas,  those  of  the  Fox  Hills  and  Horsethief  sandstones  and 
the  sandstone  at  the  top  of  the  Claggett  being  practically  identical  and  that  of  the  Virgelle  being 
a  direct  forerunner  of  later  faunas. 

Above  the  Horsethief  sandstone  in  the  section  on  Two  Medicine  River  there  are  light- 
colored  soft  clayey  and  sandy  strata,  already  referred  to  as  of  continental  origin,  that  are  identical 
in  appearance  with  the  Belly  River  and  Judith  River  strata.  Although  these  rocks  are  younger 
than  Montana  in  age,  they  deserve  mention  here  because  they  seem  to  complete  for  a  third 
time  a  cycle  of  sedimentation  proceeding  from  purely  marine  to  fresh-water  or  continental  con- 
ditions. The  first  of  these  cycles  is  from  the  marine  Colorado  shale,  through  Virgelle  sandstone, 
to  the  strata  of  continental  origin  in  the  lower  part  of  the  Two  Medicine  formation ;  the  second 
is  from  the  marine  shale  of  the  Claggett,  through  the  sandstone  in  its  upper  part  to  strata, 
also  of  continental  origin,  comprising  the  Judith  River  formation;  and  the  third  is  from  the 
marine  Bearpaw,  through  the  Horsethief  sandstone,  to  the  continental  deposits  above  that 
sandstone.  The  relations  in  each  cycle  between  the  continental  deposits  and  the  underlying 
sandstone  seem  to  be  identical.  The  strata  are  apparently  perfectly  conformable  and  the 
impression  is  very  strong  that  the  same  conditions  ruled  in  each  transition  from  marine  to  land 
conditions,  or,  in  other  words,  the  strata  above  the  Horsethief  sandstone  are  physically  as  closely 
related  to  the  Bearpaw  shale  as  the  Judith  River  formation  is  to  the  Claggett  or  the  Two  Medi- 
cine formation  to  the  Colorado  shale.  Now,  these  strata  of  continental  origin  above  the  Horse- 
thief  sandstone  constitute  the  St.  Mary  River  beds  of  Dawson,  which  occupy  the  same  position 
in  the  geologic  column  as  the  Edmonton  formation  of  central  Alberta  and  approximately  that 
of  the  Lance  formation  of  Wyoming.  The  age  of  this  formation  is  therefore  involved  in  the 
Lance-Laramie  problem  of  North  American  geology,  and  in  the  opinion  of  the  writer  the  fact 
that  in  this  part  of  the  interior  basin  this  formation,  which  seems  to  be  equivalent  to  the^ Lance, 
appears  to  be  as  intimately  related  stratigraphically  to  Cretaceous  marine  shale  below  it  as  are 
similar  continental  deposits  well  down  in  the  Cretaceous  section  to  a  corresponding  marine  shale 
should  receive  considerable  weight  in  the  consideration  of  that  problem. 

Lastly,  the  sedimentary  record  that  has  been  described  seems  noteworthy  because  it  shows 
three  successive  marked  recessions  of  a  sea  which  are  not  known  to  have  produced  unconformity 
or  other  hiatus  in  the  stratigraphy — conditions  commonly  attendant  on  such  recessions — but,  on 
the  contrary,  seem  to  have  been  followed  immediately  by  sedimentation  at  an  increased  rate, 
the  strata  deposited  bearing  conformable  relations  to  the  underlying  rocks.  The  proper  concept 
seems  to  be  that  each  recession  of  the  Upper  Cretaceous  sea  in  the  northern  interior  region  of 
North  America  was  accompanied  in  extensive  areas  of  lowlands  on  its  margins  not  by  uplift  and 
erosion  but  by  uplift  and  an  increased  rate  of  sedimentation. 

o 


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